1. Field of the Invention
The invention relates to thermally curable mixtures which consist of polyfunctional amides and aromatic di- or polyglycidylethers, or additionally a cyclic carbonate, and which are reacted by addition of a basic catalyst and are used as coatings or foams.
2. Related Art
Primary aliphatic and aromatic di- and polyamines have manifold uses as hardeners for epoxy coatings. It is especially advantageous that crosslinking takes place already at low temperatures between 0.degree. and 100.degree. C. and that because of the large choice of di- and polyamines of different chemical structure the properties of the finished coatings can be varied greatly. Disadvantageous, however, are the short pot-lives of such epoxy/amine-mixtures which reduce their storage stability and workability considerably, as well as the negative hygienic properties of many of the used amines. Also many amines show strong corrosive action, have a relatively high vapour-pressure and take up CO.sub.2 readily from the air which reduces their solubility and reactivity against epoxy compounds.
Primary- and secondary amino-functions can be blocked in many ways chemically reversible, whereby the limitations named above can be avoided at least during the manufacture, storage and use of the coatings totally or partially.
One possibility for the reversible blocking is the reaction of primary and secondary amino groups with volatile organic and inorganic acids under salt formation. The release of the reactive amino-functions occurs by addition of aqueous bases in stoichiometric amounts relative to the amino content. This possibility can be used only in aqueous coating systems in which the released acids use up epoxy groups and corrode metallic substrates, however. Another possibility for the blocking of primary amino groups is the reaction with aldehydes and ketones under formation of aldimines and ketimines which are split hydrolytically to the starting products. The most blocking agents known from peptide synthesis have to be ruled out because of their high price and the deblocking methods unusual in coatings technology such as hydrogenation, photolysis or cleavage by aggressive reagents.
Acidic and basic hydrolysable groups are secondary or tertiary amido groups, preferably formamido groups. They can be prepared in many ways, e.g. by thermolysis or dehydration with hydroscopic agents of the respective ammonium salts, by reaction of the amines with CO under pressure and noble-metal-catalysis and by reaction of amines or ammonium salts with esters, preferably formic esters. These methods, such as e.g. the thermolysis and dehydration of ammonium salts and the amidation of esters in most cases are suitable only for amines without other reactive groups.
A blocking of primary and secondary amino-functions in compounds which have additionally other reactive groups such as OH--, SH-- or ester-functions, is possible by carbonylation under pressure as described e.g. in the patents U.S. Pat. No. 4,510,326 and CS 183 083 or by reaction of ammoniumhydrochlorides with formic esters and triethylamine under acidic catalysis, as described by L. F. Tietze and Th. Eichler in the book "Reaktionen und Synthesen in organisch-chemischen Praktikum", page 120, G. Thieme Verlag, Stuttgart, New York, (1981), for aminoacidesters. The process named first requires, however, a very high technical expense, while the second process proceeds as a difficult controllable solid/liquid phase reaction, produces a large amount of non-usable triethylammoniumchloride and the final product, especially the hydroxyalkylformamide, can be separated hardly from the ammoniumchloride.